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Hawes EM, Rahim M, Haratipour Z, Orun AR, O'Rourke ML, Oeser JK, Kim K, Claxton DP, Blind RD, Young JD, O'Brien RM. Biochemical and metabolic characterization of a G6PC2 inhibitor. Biochimie 2024:S0300-9084(24)00050-6. [PMID: 38431189 DOI: 10.1016/j.biochi.2024.02.012] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2024] [Revised: 02/28/2024] [Accepted: 02/29/2024] [Indexed: 03/05/2024]
Abstract
Three glucose-6-phosphatase catalytic subunits, that hydrolyze glucose-6-phosphate (G6P) to glucose and inorganic phosphate, have been identified, designated G6PC1-3, but only G6PC1 and G6PC2 have been implicated in the regulation of fasting blood glucose (FBG). Elevated FBG has been associated with multiple adverse clinical outcomes, including increased risk for type 2 diabetes and various cancers. Therefore, G6PC1 and G6PC2 inhibitors that lower FBG may be of prophylactic value for the prevention of multiple conditions. The studies described here characterize a G6PC2 inhibitor, designated VU0945627, previously identified as Compound 3. We show that VU0945627 preferentially inhibits human G6PC2 versus human G6PC1 but activates human G6PC3. VU0945627 is a mixed G6PC2 inhibitor, increasing the Km but reducing the Vmax for G6P hydrolysis. PyRx virtual docking to an AlphaFold2-derived G6PC2 structural model suggests VU0945627 binds two sites in human G6PC2. Mutation of residues in these sites reduces the inhibitory effect of VU0945627. VU0945627 does not inhibit mouse G6PC2 despite its 84% sequence identity with human G6PC2. Mutagenesis studies suggest this lack of inhibition of mouse G6PC2 is due, in part, to a change in residue 318 from histidine in human G6PC2 to proline in mouse G6PC2. Surprisingly, VU0945627 still inhibited glucose cycling in the mouse islet-derived βTC-3 cell line. Studies using intact mouse liver microsomes and PyRx docking suggest that this observation can be explained by an ability of VU0945627 to also inhibit the G6P transporter SLC37A4. These data will inform future computational modeling studies designed to identify G6PC isoform-specific inhibitors.
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Affiliation(s)
- Emily M Hawes
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN, 37232, USA
| | - Mohsin Rahim
- Department of Chemical and Biomolecular Engineering, Vanderbilt School of Engineering, Nashville, TN, 37232, USA
| | - Zeinab Haratipour
- Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN, 37232, USA
| | - Abigail R Orun
- Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN, 37232, USA
| | - Margaret L O'Rourke
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN, 37232, USA
| | - James K Oeser
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN, 37232, USA
| | - Kwangho Kim
- Department of Chemistry, Vanderbilt Institute of Chemical Biology, Vanderbilt University, Nashville, TN, 37232, USA
| | - Derek P Claxton
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN, 37232, USA
| | - Ray D Blind
- Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN, 37232, USA
| | - Jamey D Young
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN, 37232, USA; Department of Chemical and Biomolecular Engineering, Vanderbilt School of Engineering, Nashville, TN, 37232, USA
| | - Richard M O'Brien
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN, 37232, USA.
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Sinclair M, Stein RA, Sheehan JH, Hawes EM, O’Brien RM, Tajkhorshid E, Claxton DP. Integrative analysis of pathogenic variants in glucose-6-phosphatase based on an AlphaFold2 model. PNAS Nexus 2024; 3:pgae036. [PMID: 38328777 PMCID: PMC10849595 DOI: 10.1093/pnasnexus/pgae036] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 09/01/2023] [Accepted: 01/09/2024] [Indexed: 02/09/2024]
Abstract
Mediating the terminal reaction of gluconeogenesis and glycogenolysis, the integral membrane protein glucose-6-phosphate catalytic subunit 1 (G6PC1) regulates hepatic glucose production by catalyzing hydrolysis of glucose-6-phosphate (G6P) within the lumen of the endoplasmic reticulum. Consistent with its vital contribution to glucose homeostasis, inactivating mutations in G6PC1 causes glycogen storage disease (GSD) type 1a characterized by hepatomegaly and severe hypoglycemia. Despite its physiological importance, the structural basis of G6P binding to G6PC1 and the molecular disruptions induced by missense mutations within the active site that give rise to GSD type 1a are unknown. In this study, we determine the atomic interactions governing G6P binding as well as explore the perturbations imposed by disease-linked missense variants by subjecting an AlphaFold2 G6PC1 structural model to molecular dynamics simulations and in silico predictions of thermodynamic stability validated with robust in vitro and in situ biochemical assays. We identify a collection of side chains, including conserved residues from the signature phosphatidic acid phosphatase motif, that contribute to a hydrogen bonding and van der Waals network stabilizing G6P in the active site. The introduction of GSD type 1a mutations modified the thermodynamic landscape, altered side chain packing and substrate-binding interactions, and induced trapping of catalytic intermediates. Our results, which corroborate the high quality of the AF2 model as a guide for experimental design and to interpret outcomes, not only confirm the active-site structural organization but also identify previously unobserved mechanistic contributions of catalytic and noncatalytic side chains.
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Affiliation(s)
- Matt Sinclair
- Theoretical and Computational Biophysics Group, NIH Center for Macromolecular Modeling and Visualization, Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
- Department of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
| | - Richard A Stein
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN 37232, USA
- Center for Applied Artificial Intelligence in Protein Dynamics, Vanderbilt University, Nashville, TN 37240, USA
| | - Jonathan H Sheehan
- Center for Structural Biology, Vanderbilt University, Nashville, TN 37240, USA
- Division of Infectious Diseases, Department of Internal Medicine, Washington University School of Medicine, St Louis, MO 63110, USA
| | - Emily M Hawes
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN 37232, USA
| | - Richard M O’Brien
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN 37232, USA
| | - Emad Tajkhorshid
- Theoretical and Computational Biophysics Group, NIH Center for Macromolecular Modeling and Visualization, Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
- Department of Biochemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
- Center for Biophysics and Quantitative Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
| | - Derek P Claxton
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN 37232, USA
- Center for Applied Artificial Intelligence in Protein Dynamics, Vanderbilt University, Nashville, TN 37240, USA
- Center for Structural Biology, Vanderbilt University, Nashville, TN 37240, USA
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3
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Sinclair M, Stein RA, Sheehan JH, Hawes EM, O'Brien RM, Tajkhorshid E, Claxton DP. Molecular mechanisms of catalytic inhibition for active site mutations in glucose-6-phosphatase catalytic subunit 1 linked to glycogen storage disease. bioRxiv 2023:2023.03.13.532485. [PMID: 36993754 PMCID: PMC10054992 DOI: 10.1101/2023.03.13.532485] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/19/2023]
Abstract
Mediating the terminal reaction of gluconeogenesis and glycogenolysis, the integral membrane protein G6PC1 regulates hepatic glucose production by catalyzing hydrolysis of glucose-6-phosphate (G6P) within the lumen of the endoplasmic reticulum. Consistent with its vital contribution to glucose homeostasis, inactivating mutations in G6PC1 cause glycogen storage disease (GSD) type 1a characterized by hepatomegaly and severe hypoglycemia. Despite its physiological importance, the structural basis of G6P binding to G6PC1 and the molecular disruptions induced by missense mutations within the active site that give rise to GSD type 1a are unknown. Exploiting a computational model of G6PC1 derived from the groundbreaking structure prediction algorithm AlphaFold2 (AF2), we combine molecular dynamics (MD) simulations and computational predictions of thermodynamic stability with a robust in vitro screening platform to define the atomic interactions governing G6P binding as well as explore the energetic perturbations imposed by disease-linked variants. We identify a collection of side chains, including conserved residues from the signature phosphatidic acid phosphatase motif, that contribute to a hydrogen bonding and van der Waals network stabilizing G6P in the active site. Introduction of GSD type 1a mutations into the G6PC1 sequence elicits changes in G6P binding energy, thermostability and structural properties, suggesting multiple pathways of catalytic impairment. Our results, which corroborate the high quality of the AF2 model as a guide for experimental design and to interpret outcomes, not only confirm active site structural organization but also suggest novel mechanistic contributions of catalytic and non-catalytic side chains.
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Sinclair M, Sheehan J, Stein RA, Claxton DP, Tajkhorshid E. Simulations reveal the role of glucose-6-phosphatase mutations in glycogen storage disease. Biophys J 2023; 122:185a. [PMID: 36782880 DOI: 10.1016/j.bpj.2022.11.1141] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/12/2023] Open
Affiliation(s)
- Matt Sinclair
- Department of Biochemistry, Center for Biophysics and Quantitative Biology, University of Illinois Urbana-Champaign, Urbana, IL, USA; Theoretical and Computational Biophysics Group, National Institutes of Health Center for Macromolecular Modeling and Bioinformatics, Beckman Institute for Advanced Science and Technology, University of Illinois Urbana-Champaign, Urbana, IL, USA
| | | | - Richard A Stein
- Department of Molecular Physiology & Biophysics, Vanderbilt University, Nashville, TN, USA
| | - Derek P Claxton
- Department of Molecular Physiology & Biophysics, Vanderbilt University, Nashville, TN, USA
| | - Emad Tajkhorshid
- Department of Biochemistry, Center for Biophysics and Quantitative Biology, University of Illinois Urbana-Champaign, Urbana, IL, USA; Theoretical and Computational Biophysics Group, National Institutes of Health Center for Macromolecular Modeling and Bioinformatics, Beckman Institute for Advanced Science and Technology, University of Illinois Urbana-Champaign, Urbana, IL, USA
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5
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Overway EM, Bosma KJ, Claxton DP, Oeser JK, Singh K, Breidenbach LB, Mchaourab HS, Davis LK, O'Brien RM. Nonsynonymous single-nucleotide polymorphisms in the G6PC2 gene affect protein expression, enzyme activity, and fasting blood glucose. J Biol Chem 2022; 298:101534. [PMID: 34954144 PMCID: PMC8800118 DOI: 10.1016/j.jbc.2021.101534] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/27/2021] [Revised: 12/16/2021] [Accepted: 12/17/2021] [Indexed: 12/30/2022] Open
Abstract
G6PC2 encodes a glucose-6-phosphatase (G6Pase) catalytic subunit that modulates the sensitivity of insulin secretion to glucose and thereby regulates fasting blood glucose (FBG). A common single-nucleotide polymorphism (SNP) in G6PC2, rs560887 is an important determinant of human FBG variability. This SNP has a subtle effect on G6PC2 RNA splicing, which raises the question as to whether nonsynonymous SNPs with a major impact on G6PC2 stability or enzyme activity might have a broader disease/metabolic impact. Previous attempts to characterize such SNPs were limited by the very low inherent G6Pase activity and expression of G6PC2 protein in islet-derived cell lines. In this study, we describe the use of a plasmid vector that confers high G6PC2 protein expression in islet cells, allowing for a functional analysis of 22 nonsynonymous G6PC2 SNPs, 19 of which alter amino acids that are conserved in mouse G6PC2 and the human and mouse variants of the related G6PC1 isoform. We show that 16 of these SNPs markedly impair G6PC2 protein expression (>50% decrease). These SNPs have variable effects on the stability of human and mouse G6PC1, despite the high sequence homology between these isoforms. Four of the remaining six SNPs impaired G6PC2 enzyme activity. Electronic health record-derived phenotype analyses showed an association between high-impact SNPs and FBG, but not other diseases/metabolites. While homozygous G6pc2 deletion in mice increases the risk of hypoglycemia, these human data reveal no evidence that the beneficial use of partial G6PC2 inhibitors to lower FBG would be associated with unintended negative consequences.
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Affiliation(s)
- Emily M Overway
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA
| | - Karin J Bosma
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA
| | - Derek P Claxton
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA
| | - James K Oeser
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA
| | - Kritika Singh
- Department of Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee, USA
| | - Lindsay B Breidenbach
- Department of Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee, USA
| | - Hassane S Mchaourab
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA
| | - Lea K Davis
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA; Department of Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee, USA
| | - Richard M O'Brien
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA.
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6
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Claxton DP, Overway EM, Oeser JK, O'Brien RM, Mchaourab HS. Biophysical and functional properties of purified glucose-6-phosphatase catalytic subunit 1. J Biol Chem 2021; 298:101520. [PMID: 34952005 PMCID: PMC8753184 DOI: 10.1016/j.jbc.2021.101520] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2021] [Revised: 12/10/2021] [Accepted: 12/17/2021] [Indexed: 11/18/2022] Open
Abstract
Glucose-6-phosphatase catalytic subunit 1 (G6PC1) plays a critical role in hepatic glucose production during fasting by mediating the terminal step of the gluconeogenesis and glycogenolysis pathways. In concert with accessory transport proteins, this membrane-integrated enzyme catalyzes glucose production from glucose-6-phosphate (G6P) to support blood glucose homeostasis. Consistent with its metabolic function, dysregulation of G6PC1 gene expression contributes to diabetes, and mutations that impair phosphohydrolase activity form the clinical basis of glycogen storage disease type 1a. Despite its relevance to health and disease, a comprehensive view of G6PC1 structure and mechanism has been limited by the absence of expression and purification strategies that isolate the enzyme in a functional form. In this report, we apply a suite of biophysical and biochemical tools to fingerprint the in vitro attributes of catalytically active G6PC1 solubilized in lauryl maltose neopentyl glycol (LMNG) detergent micelles. When purified from Sf9 insect cell membranes, the glycosylated mouse ortholog (mG6PC1) recapitulated functional properties observed previously in intact hepatic microsomes and displayed the highest specific activity reported to date. Additionally, our results establish a direct correlation between the catalytic and structural stability of mG6PC1, which is underscored by the enhanced thermostability conferred by phosphatidylcholine and the cholesterol analog cholesteryl hemisuccinate. In contrast, the N96A variant, which blocks N-linked glycosylation, reduced thermostability. The methodologies described here overcome long-standing obstacles in the field and lay the necessary groundwork for a detailed analysis of the mechanistic structural biology of G6PC1 and its role in complex metabolic disorders.
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Affiliation(s)
- Derek P Claxton
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee, USA.
| | - Emily M Overway
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee, USA
| | - James K Oeser
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee, USA
| | - Richard M O'Brien
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee, USA
| | - Hassane S Mchaourab
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee, USA
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7
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Claxton DP, Jagessar KL, Mchaourab HS. Principles of Alternating Access in Multidrug and Toxin Extrusion (MATE) Transporters. J Mol Biol 2021; 433:166959. [PMID: 33774036 DOI: 10.1016/j.jmb.2021.166959] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2021] [Revised: 03/15/2021] [Accepted: 03/18/2021] [Indexed: 12/18/2022]
Abstract
The multidrug and toxin extrusion (MATE) transporters catalyze active efflux of a broad range of chemically- and structurally-diverse compounds including antimicrobials and chemotherapeutics, thus contributing to multidrug resistance in pathogenic bacteria and cancers. Multiple methodological approaches have been taken to investigate the structural basis of energy transduction and substrate translocation in MATE transporters. Crystal structures representing members from all three MATE subfamilies have been interpreted within the context of an alternating access mechanism that postulates occupation of distinct structural intermediates in a conformational cycle powered by electrochemical ion gradients. Here we review the structural biology of MATE transporters, integrating the crystallographic models with biophysical and computational studies to define the molecular determinants that shape the transport energy landscape. This holistic analysis highlights both shared and disparate structural and functional features within the MATE family, which underpin an emerging theme of mechanistic diversity within the framework of a conserved structural scaffold.
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Affiliation(s)
- Derek P Claxton
- Department of Molecular Physiology and Biophysics, Vanderbilt University, 747 Light Hall, 2215 Garland Avenue, Nashville, TN 37232, USA
| | - Kevin L Jagessar
- Department of Molecular Physiology and Biophysics, Vanderbilt University, 747 Light Hall, 2215 Garland Avenue, Nashville, TN 37232, USA
| | - Hassane S Mchaourab
- Department of Molecular Physiology and Biophysics, Vanderbilt University, 747 Light Hall, 2215 Garland Avenue, Nashville, TN 37232, USA.
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8
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Han B, Porta JC, Hanks JL, Peskova Y, Binshtein E, Dryden KA, Claxton DP, Mchaourab HS, Karakas E, Ohi MD, Kenworthy AK. Single Particle Analysis Reveals the Organization of the Membrane Remodeling Protein Caveolin-1 within Disc-Shaped Complexes. Biophys J 2021. [DOI: 10.1016/j.bpj.2020.11.1873] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022] Open
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9
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Castellano S, Claxton DP, Ficici E, Kusakizako T, Stix R, Zhou W, Nureki O, Mchaourab HS, Faraldo-Gómez JD. Conserved binding site in the N-lobe of prokaryotic MATE transporters suggests a role for Na + in ion-coupled drug efflux. J Biol Chem 2021:jbc.RA120.016792. [PMID: 33402425 DOI: 10.1074/jbc.ra120.016792] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2020] [Accepted: 01/05/2021] [Indexed: 12/14/2022] Open
Abstract
In both prokaryotes and eukaryotes, multidrug and toxic-compound extrusion (MATE) transporters catalyze the efflux of a broad range of cytotoxic compounds, including human-made antibiotics and anticancer drugs. MATEs are secondary-active antiporters, i.e. their drug-efflux activity is coupled to, and powered by, the uptake of ions down a pre-existing transmembrane electrochemical gradient. Key aspects of this mechanism, however, remain to be delineated, such as its ion specificity and stoichiometry. We previously revealed the existence of a Na+-binding site in a MATE transporter from Pyroccocus furiosus (PfMATE) and hypothesized that this site might be broadly conserved among prokaryotic MATEs. Here, we evaluate this hypothesis by analyzing VcmN and ClbM, which along with PfMATE are the only three prokaryotic MATEs whose molecular structures have been determined at resolutions better than 3 Å. Analysis of available crystallographic data and molecular dynamics simulations indeed reveal an occupied Na+-binding site in the N-terminal lobe of both structures, analogous to that identified in PfMATE. We likewise find this site to be strongly selective against K+, suggesting it is mechanistically significant. Consistent with these computational results, DEER spectroscopy measurements for multiple doubly-spin-labeled VcmN constructs demonstrate Na+-dependent changes in protein conformation. The existence of this binding site in three MATE orthologs implicates Na+ in the ion-coupled drug-efflux mechanisms of this class of transporters. These results also imply that observations of H+-dependent activity stem either from a site elsewhere in the structure, or from H+ displacing Na+ under certain laboratory conditions, as has been noted for other Na+-driven transport systems.
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Affiliation(s)
| | | | - Emel Ficici
- National Institutes of Health, United States
| | | | - Robyn Stix
- National Institutes of Health, United States
| | | | | | - Hassane S Mchaourab
- Dept of Molecular Physiology and Biophysics, Vanderbilt University, United States
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10
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Han B, Porta JC, Hanks JL, Peskova Y, Binshtein E, Dryden K, Claxton DP, Mchaourab HS, Karakas E, Ohi MD, Kenworthy AK. Structure and assembly of CAV1 8S complexes revealed by single particle electron microscopy. Sci Adv 2020; 6:6/49/eabc6185. [PMID: 33268374 PMCID: PMC7821874 DOI: 10.1126/sciadv.abc6185] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/08/2020] [Accepted: 10/16/2020] [Indexed: 05/21/2023]
Abstract
Highly stable oligomeric complexes of the monotopic membrane protein caveolin serve as fundamental building blocks of caveolae. Current evidence suggests these complexes are disc shaped, but the details of their structural organization and how they assemble are poorly understood. Here, we address these questions using single particle electron microscopy of negatively stained recombinant 8S complexes of human caveolin 1. We show that 8S complexes are toroidal structures ~15 nm in diameter that consist of an outer ring, an inner ring, and central protruding stalk. Moreover, we map the position of the N and C termini and determine their role in complex assembly, and visualize the 8S complexes in heterologous caveolae. Our findings provide critical insights into the structural features of 8S complexes and allow us to propose a model for how these highly stable membrane-embedded complexes are generated.
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Affiliation(s)
- Bing Han
- Center for Membrane and Cell Physiology, University of Virginia, Charlottesville, VA, USA
- Department of Molecular Physiology and Biological Physics, University of Virginia School of Medicine, Charlottesville, VA, USA
| | - Jason C Porta
- Life Sciences Institute, University of Michigan, Ann Arbor, MI, USA
| | - Jessica L Hanks
- Life Sciences Institute, University of Michigan, Ann Arbor, MI, USA
| | - Yelena Peskova
- Center for Membrane and Cell Physiology, University of Virginia, Charlottesville, VA, USA
- Department of Molecular Physiology and Biological Physics, University of Virginia School of Medicine, Charlottesville, VA, USA
| | - Elad Binshtein
- Center for Structural Biology, Vanderbilt University, Nashville, TN, USA
| | - Kelly Dryden
- Department of Molecular Physiology and Biological Physics, University of Virginia School of Medicine, Charlottesville, VA, USA
| | - Derek P Claxton
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN, USA
| | - Hassane S Mchaourab
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN, USA
| | - Erkan Karakas
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN, USA
| | - Melanie D Ohi
- Life Sciences Institute, University of Michigan, Ann Arbor, MI, USA.
- Department of Cell and Developmental Biology, University of Michigan School of Medicine, Ann Arbor, MI, USA
| | - Anne K Kenworthy
- Center for Membrane and Cell Physiology, University of Virginia, Charlottesville, VA, USA.
- Department of Molecular Physiology and Biological Physics, University of Virginia School of Medicine, Charlottesville, VA, USA
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11
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Jagessar KL, Mchaourab HS, Claxton DP. The N-terminal domain of an archaeal multidrug and toxin extrusion (MATE) transporter mediates proton coupling required for prokaryotic drug resistance. J Biol Chem 2019; 294:12807-12814. [PMID: 31289123 DOI: 10.1074/jbc.ra119.009195] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2019] [Revised: 07/05/2019] [Indexed: 12/15/2022] Open
Abstract
As a contributor to multidrug resistance, the family of multidrug and toxin extrusion (MATE) transporters couples the efflux of chemically dissimilar compounds to electrochemical ion gradients. Although divergent transport mechanisms have been proposed for these transporters, previous structural and functional analyses of members of the MATE subfamily DinF suggest that the N-terminal domain (NTD) supports substrate and ion binding. In this report, we investigated the relationship of ligand binding within the NTD to the drug resistance mechanism of the H+-dependent MATE from the hyperthermophilic archaeon Pyrococcus furiosus (PfMATE). To facilitate this study, we developed a cell growth assay in Escherichia coli to characterize the resistance conferred by PfMATE to toxic concentrations of the antimicrobial compound rhodamine 6G. Expression of WT PfMATE promoted cell growth in the presence of drug, but amino acid substitutions of conserved NTD residues compromised drug resistance. Steady-state binding analysis with purified PfMATE indicated that substrate affinity was unperturbed in these NTD variants. However, exploiting Trp fluorescence as an intrinsic reporter of conformational changes, we found that these variants impaired formation of a unique H+-stabilized structural intermediate. These results imply that disruption of H+ coupling is the origin of compromised toxin resistance in PfMATE variants. These findings support a model mechanism wherein the NTD mediates allosteric coupling to ion gradients through conformational changes to drive substrate transport in PfMATE. Furthermore, the results provide evidence for diverging transport mechanisms within a prokaryotic MATE subfamily.
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Affiliation(s)
- Kevin L Jagessar
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232
| | - Hassane S Mchaourab
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232
| | - Derek P Claxton
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232
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12
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Phulera S, Zhu H, Yu J, Claxton DP, Yoder N, Yoshioka C, Gouaux E. Cryo-EM structure of the benzodiazepine-sensitive α1β1γ2S tri-heteromeric GABA A receptor in complex with GABA. eLife 2018; 7:39383. [PMID: 30044221 PMCID: PMC6086659 DOI: 10.7554/elife.39383] [Citation(s) in RCA: 91] [Impact Index Per Article: 15.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/20/2018] [Accepted: 07/24/2018] [Indexed: 12/12/2022] Open
Abstract
Fast inhibitory neurotransmission in the mammalian nervous system is largely mediated by GABAA receptors, chloride-selective members of the superfamily of pentameric Cys-loop receptors. Native GABAA receptors are heteromeric assemblies sensitive to many important drugs, from sedatives to anesthetics and anticonvulsant agents, with mutant forms of GABAA receptors implicated in multiple neurological diseases. Despite the profound importance of heteromeric GABAA receptors in neuroscience and medicine, they have proven recalcitrant to structure determination. Here we present the structure of a tri-heteromeric α1β1γ2SEM GABAA receptor in complex with GABA, determined by single particle cryo-EM at 3.1–3.8 Å resolution, elucidating molecular principles of receptor assembly and agonist binding. Remarkable N-linked glycosylation on the α1 subunit occludes the extracellular vestibule of the ion channel and is poised to modulate receptor assembly and perhaps ion channel gating. Our work provides a pathway to structural studies of heteromeric GABAA receptors and a framework for rational design of novel therapeutic agents.
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Affiliation(s)
- Swastik Phulera
- Vollum Institute, Oregon Health and Science University, Portland, United States
| | - Hongtao Zhu
- Vollum Institute, Oregon Health and Science University, Portland, United States
| | - Jie Yu
- Vollum Institute, Oregon Health and Science University, Portland, United States
| | - Derek P Claxton
- Vollum Institute, Oregon Health and Science University, Portland, United States
| | - Nate Yoder
- Vollum Institute, Oregon Health and Science University, Portland, United States
| | - Craig Yoshioka
- Vollum Institute, Oregon Health and Science University, Portland, United States
| | - Eric Gouaux
- Vollum Institute, Oregon Health and Science University, Portland, United States.,Howard Hughes Medical Institute, Oregon Health and Science University, Portland, United States
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13
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Abstract
The GABA-gated chloride channels of the Cys-loop receptor family, known as GABAA receptors, function as the primary gatekeepers of fast inhibitory neurotransmission in the central nervous system. Formed by the pentameric arrangement of five identical or homologous subunits, GABAA receptor subtypes are defined by the subunit composition that shape ion channel properties. An understanding of the structural basis of distinct receptor properties has been hindered by the absence of high resolution structural information for heteromeric assemblies. Robust heterologous expression and purification protocols of high expressing receptor constructs are vital for structural studies. Here, we describe a unique approach to screen for well-behaving and functional GABAA receptor subunit assemblies by using the Xenopus oocyte as an expression host in combination with fluorescence detection size exclusion chromatography (FSEC). To detect receptor expression, GFP fusions were introduced into the α1 subunit isoform. In contrast to expression of α1 alone, co-expression with the β subunit promoted formation of monodisperse assemblies. Mutagenesis experiments suggest that the α and β subunits can tolerate large truncations in the non-conserved M3/M4 cytoplasmic loop without compromising oligomeric assembly or GABA-gated channel activity, although removal of N-linked glycosylation sites is negatively correlated with expression level. Additionally, we report methods to improve GABAA receptor expression in mammalian cell culture that employ recombinant baculovirus transduction. From these methods we have identified a well-behaving minimal functional construct for the α1/β1 GABAA receptor subtype that can be purified in milligram quantities while retaining high affinity agonist binding activity.
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Affiliation(s)
- Derek P. Claxton
- Vollum Institute, Oregon Health and Science University, Portland, Oregon, United States of America
| | - Eric Gouaux
- Vollum Institute, Oregon Health and Science University, Portland, Oregon, United States of America
- Howard Hughes Medical Institute, Oregon Health and Science University, Portland, Oregon, United States of America
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14
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Mishra S, Chandler SA, Williams D, Claxton DP, Koteiche HA, Stewart PL, Benesch JLP, Mchaourab HS. Engineering of a Polydisperse Small Heat-Shock Protein Reveals Conserved Motifs of Oligomer Plasticity. Structure 2018; 26:1116-1126.e4. [PMID: 29983375 DOI: 10.1016/j.str.2018.05.015] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2017] [Revised: 04/10/2018] [Accepted: 05/18/2018] [Indexed: 01/21/2023]
Abstract
Small heat-shock proteins (sHSPs) are molecular chaperones that bind partially and globally unfolded states of their client proteins. Previously, we discovered that the archaeal Hsp16.5, which forms ordered and symmetric 24-subunit oligomers, can be engineered to transition to an ordered and symmetric 48-subunit oligomer by insertion of a peptide from human HspB1 (Hsp27). Here, we uncovered the existence of an array of oligomeric states (30-38 subunits) that can be populated as a consequence of altering the sequence and length of the inserted peptide. Polydisperse Hsp16.5 oligomers displayed higher affinity to a model client protein consistent with a general mechanism for recognition and binding that involves increased access of the hydrophobic N-terminal region. Our findings, which integrate structural and functional analyses from evolutionarily distant sHSPs, support a model wherein the modular architecture of these proteins encodes motifs of oligomer polydispersity, dissociation, and expansion to achieve functional diversity and regulation.
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Affiliation(s)
- Sanjay Mishra
- Chemical & Physical Biology Program, Vanderbilt University, Nashville 37232, TN, USA; Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville 37232, TN, USA
| | - Shane A Chandler
- Department of Chemistry, University of Oxford, Oxford OX1 3TA, UK
| | - Dewight Williams
- John M. Cowley Center for High Resolution Electron Microscopy, Arizona State University, Tempe 85287, AZ, USA
| | - Derek P Claxton
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville 37232, TN, USA
| | - Hanane A Koteiche
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville 37232, TN, USA
| | - Phoebe L Stewart
- Department of Pharmacology Case Western Reserve University, Cleveland, OH 44106, USA
| | | | - Hassane S Mchaourab
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville 37232, TN, USA.
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15
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Paz A, Claxton DP, Kumar JP, Kazmier K, Bisignano P, Sharma S, Nolte SA, Liwag TM, Nayak V, Wright EM, Grabe M, Mchaourab HS, Abramson J. Conformational transitions of the sodium-dependent sugar transporter, vSGLT. Proc Natl Acad Sci U S A 2018; 115:E2742-E2751. [PMID: 29507231 PMCID: PMC5866573 DOI: 10.1073/pnas.1718451115] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
Sodium-dependent transporters couple the flow of Na+ ions down their electrochemical potential gradient to the uphill transport of various ligands. Many of these transporters share a common core structure composed of a five-helix inverted repeat and deliver their cargo utilizing an alternating-access mechanism. A detailed characterization of inward-facing conformations of the Na+-dependent sugar transporter from Vibrio parahaemolyticus (vSGLT) has previously been reported, but structural details on additional conformations and on how Na+ and ligand influence the equilibrium between other states remains unknown. Here, double electron-electron resonance spectroscopy, structural modeling, and molecular dynamics are utilized to deduce ligand-dependent equilibria shifts of vSGLT in micelles. In the absence and presence of saturating amounts of Na+, vSGLT favors an inward-facing conformation. Upon binding both Na+ and sugar, the equilibrium shifts toward either an outward-facing or occluded conformation. While Na+ alone does not stabilize the outward-facing state, gating charge calculations together with a kinetic model of transport suggest that the resting negative membrane potential of the cell, absent in detergent-solubilized samples, may stabilize vSGLT in an outward-open conformation where it is poised for binding external sugars. In total, these findings provide insights into ligand-induced conformational selection and delineate the transport cycle of vSGLT.
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Affiliation(s)
- Aviv Paz
- Department of Physiology, David Geffen School of Medicine, University of California, Los Angeles, CA 90096
| | - Derek P Claxton
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN 37232
| | - Jay Prakash Kumar
- Technologies for the Advancement of Science, Institute for Stem Cell Biology and Regenerative Medicine, 560065, Bangalore, India
- School of Life Science, The Institute of TransDisciplinary Health Sciences & Technology (TDU), 560064, Bangalore, India
| | - Kelli Kazmier
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN 37232
| | - Paola Bisignano
- Cardiovascular Research Institute, University of California, San Francisco, CA 94158
- Department of Pharmaceutical Chemistry, University of California, San Francisco, CA 94158
| | - Shruti Sharma
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN 37232
| | - Shannon A Nolte
- Molecular Cell and Developmental Biology Program, University of California, Los Angeles, CA 90095
| | - Terrin M Liwag
- Molecular Cell and Developmental Biology Program, University of California, Los Angeles, CA 90095
| | - Vinod Nayak
- Technologies for the Advancement of Science, Institute for Stem Cell Biology and Regenerative Medicine, 560065, Bangalore, India
| | - Ernest M Wright
- Department of Physiology, David Geffen School of Medicine, University of California, Los Angeles, CA 90096;
| | - Michael Grabe
- Cardiovascular Research Institute, University of California, San Francisco, CA 94158;
- Department of Pharmaceutical Chemistry, University of California, San Francisco, CA 94158
| | - Hassane S Mchaourab
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN 37232;
| | - Jeff Abramson
- Department of Physiology, David Geffen School of Medicine, University of California, Los Angeles, CA 90096;
- Technologies for the Advancement of Science, Institute for Stem Cell Biology and Regenerative Medicine, 560065, Bangalore, India
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16
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Paz A, Claxton DP, Sharma S, Kazmier K, Prakash Kumar J, Nolte SA, Liwag TM, Mchaourab HS, Abramson J. Revealing the Transport Cycle Dynamics of the Sodium Dependent Sugar Transporter by Double Electron-Electron Resonance and Wide-Angle X-ray Scattering. Biophys J 2016. [DOI: 10.1016/j.bpj.2015.11.1228] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
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17
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Koteiche HA, Claxton DP, Mishra S, Stein RA, McDonald ET, Mchaourab HS. Species-Specific Structural and Functional Divergence of α-Crystallins: Zebrafish αBa- and Rodent αA(ins)-Crystallin Encode Activated Chaperones. Biochemistry 2015; 54:5949-58. [PMID: 26378715 DOI: 10.1021/acs.biochem.5b00678] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
In addition to contributing to lens optical properties, the α-crystallins are small heat shock proteins that possess chaperone activity and are predicted to bind and sequester destabilized proteins to delay cataract formation. The current model of α-crystallin chaperone mechanism envisions a transition from the native oligomer to an activated form that has higher affinity to non-native states of the substrate. Previous studies have suggested that this oligomeric plasticity is encoded in the primary sequence and controls access to high affinity binding sites within the N-terminal domain. Here, we further examined the role of sequence variation in the context of species-specific α-crystallins from rat and zebrafish. Alternative splicing of the αA gene in rodents produces αA(ins), which is distinguished by a longer N-terminal domain. The zebrafish genome includes duplicate αB-crystallin genes, αBa and αBb, which display divergent primary sequence and tissue expression patterns. Equilibrium binding experiments were employed to quantitatively define chaperone interactions with a destabilized model substrate, T4 lysozyme. In combination with multiangle light scattering, we show that rat αA(ins) and zebrafish α-crystallins display distinct global structural properties and chaperone activities. Notably, we find that αA(ins) and αBa demonstrate substantially enhanced chaperone function relative to other α-crystallins, binding the same substrate more than 2 orders of magnitude higher affinity and mimicking the activity of fully activated mammalian small heat shock proteins. These results emphasize the role of sequence divergence as an evolutionary strategy to tune chaperone function to the requirements of the tissues and organisms in which they are expressed.
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Affiliation(s)
- Hanane A Koteiche
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine , Nashville, Tennessee 37232, United States
| | - Derek P Claxton
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine , Nashville, Tennessee 37232, United States
| | - Sanjay Mishra
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine , Nashville, Tennessee 37232, United States
| | - Richard A Stein
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine , Nashville, Tennessee 37232, United States
| | - Ezelle T McDonald
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine , Nashville, Tennessee 37232, United States
| | - Hassane S Mchaourab
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine , Nashville, Tennessee 37232, United States
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18
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Claxton DP, Kazmier K, Mishra S, Mchaourab HS. Navigating Membrane Protein Structure, Dynamics, and Energy Landscapes Using Spin Labeling and EPR Spectroscopy. Methods Enzymol 2015; 564:349-87. [PMID: 26477257 DOI: 10.1016/bs.mie.2015.07.026] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
A detailed understanding of the functional mechanism of a protein entails the characterization of its energy landscape. Achieving this ambitious goal requires the integration of multiple approaches including determination of high-resolution crystal structures, uncovering conformational sampling under distinct biochemical conditions, characterizing the kinetics and thermodynamics of transitions between functional intermediates using spectroscopic techniques, and interpreting and harmonizing the data into novel computational models. With increasing sophistication in solution-based and ensemble-oriented biophysical approaches such as electron paramagnetic resonance (EPR) spectroscopy, atomic resolution structural information can be directly linked to conformational sampling in solution. Here, we detail how recent methodological and technological advances in EPR spectroscopy have contributed to the elucidation of membrane protein mechanisms. Furthermore, we aim to assist investigators interested in pursuing EPR studies by providing an introduction to the technique, a primer on experimental design, and a description of the practical considerations of the method toward generating high quality data.
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Affiliation(s)
- Derek P Claxton
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA.
| | - Kelli Kazmier
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA
| | - Smriti Mishra
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA
| | - Hassane S Mchaourab
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee, USA.
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19
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Claxton DP, Zou P, Mchaourab HS. Structure and orientation of T4 lysozyme bound to the small heat shock protein alpha-crystallin. J Mol Biol 2007; 375:1026-39. [PMID: 18062989 DOI: 10.1016/j.jmb.2007.11.014] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2007] [Revised: 10/17/2007] [Accepted: 11/06/2007] [Indexed: 10/22/2022]
Abstract
We have determined the structural changes that accompany the formation of a stable complex between a destabilized mutant of T4 lysozyme (T4L) and the small heat shock protein alpha-crystallin. Using pairs of fluorescence or spin label probes to fingerprint the T4L tertiary fold, we demonstrate that binding disrupts tertiary packing in the two domains as well as across the active-site cleft. Furthermore, increased distances between i and i+4 residues of helices support a model in which the bound structure is not native-like but significantly unfolded. In the confines of the oligomer, T4L has a preferential orientation with residues in the more hydrophobic C-terminal domain sequestered in a buried environment, while residues in the N-terminal domain are exposed to the aqueous solvent. Furthermore, electron paramagnetic resonance spectral line shapes of sites in the N-terminal domain are narrower than in the folded, unbound T4L reflecting an unstructured backbone and an asymmetric pattern of contacts between T4L and alpha-crystallin. The net orientation is not affected by the location of the destabilizing mutation consistent with the notion that binding is not triggered by recognition of localized unfolding. Together, the structural and thermodynamic data indicate that the stably bound conformation of T4L is unfolded and support a model in which the two modes of substrate binding originate from two discrete binding sites on the chaperone.
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Affiliation(s)
- Derek P Claxton
- Department of Molecular Physiology and Biophysics, Vanderbilt University Medical Center, 741 Light Hall, 2215 Garland Avenue, Nashville, TN 37232, USA
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20
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Gavira J JA, Claxton DP, Wang JH, Toh D, Meehan EJ, DiGiammarino EL. Purification, crystallization and preliminary X-ray analysis of Caenorhabditis elegans ubiquitin-conjugation enzyme M7.1. Acta Crystallogr D Biol Crystallogr 2003; 59:544-6. [PMID: 12595721 DOI: 10.1107/s0907444903000039] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/22/2002] [Accepted: 01/02/2003] [Indexed: 11/10/2022]
Abstract
M7.1 is a class IV ubiquitin-conjugation enzyme (UBC) that belongs to the ubiquitination cascade in Caenorhabditis elegans. The clone for this UBC has been overexpressed in Escherichia coli and the 16.7 kDa protein was purified from the soluble fraction. M7.1 was crystallized by sitting-drop vapor diffusion in 10% ethanol, 1.5 M NaCl at 277.5 K. Crystals diffracted to 1.75 A and belong to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 44.3, b = 54.3, c = 60.2 A. The asymmetric unit contains a single monomer. A molecular-replacement model has been determined and refinement is in progress.
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Affiliation(s)
- José A Gavira J
- Laboratory for Structural Biology, The University of Alabama in Huntsville, Huntsville, Alabama, USA
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